The present invention relates generally to class D amplifiers and, more particularly, to class D amplifiers that have one or modes of operation for avoiding AM radio harmonic frequencies during operation.
Class D power amplifiers are typically pulse-width modulated amplifiers that switch at frequencies well above the top of the audio band, often at frequencies of 100 kHz or greater. When a class D amplifier switches at these high frequencies, the switching frequency or its harmonics can interfere with AM radio receivers that are located close to the class D amplifier. Because of these interference problems, class D amplifiers cannot be easily integrated into consumer electronic products, such as stereo receivers, that have an AM tuner and power amplifier in the same chassis. Class D modulators switching in the 50 khz to 2 MHz range generate harmonics which interfere with AM radio reception. This has precluded wide spread acceptance of class D in products with an AM radio.
The AM radio broadcast band spans from 540 to 1700 kHz in the US and up to 30 MHz worldwide. To sample a 20 khz audio signal, class D modulators must run at frequencies greater than 200 khz. Because the output of these modulators is a pulse width modulated square wave, the modulators generate both even and odd harmonics. The low pass filter that removes the carrier from the speaker leads also attenuates these harmonics. However, it is not practical to design a filter with adequate high frequency attenuation and still pass 20 kHz audio signals without interfering with the sensitive AM receiver bandpass. Furthermore, the printed circuit board traces with the pulse width modulated square wave radiate. This radiation can be picked up directly by the AM antenna.
In theory the problem can be solved by ensuring the clock frequency of the class D modulator is much higher than the AM broadcast band. This however cannot be practically implemented for several reasons: 1) With a 2 MHz carrier the FETs must be switched by high current gate drivers. At the duty cycle extremes, the very short on and off times are not possible to achieve even with high gate drive. Thus, the theoretical power is limited. 2) The fast switching times will make it nearly impossible to achieve EMC compliance above 30 Mhz. 3) Unless all the clocks are synchronized in stereo and five channel applications, IMD products will be generated that will interfere with the AM band. 4) The body diodes of the MOSFETS with their long recovery time, cannot be used at this high frequency. Thus, a Shottky commutating diode is required. At bus voltages greater than 48 VDC, the forward drop of this diode may be higher than that of the body diode, and the body diode will have to be blocked with a drain diode. 5) The AM band in Europe extends to 30 Mhz.
The present invention provides, in one form thereof, circuits and methods for solving the problem of class D amplifier interference with the AM radio band. In its broader aspects the invention provides one or more reference standards for frequency. The AM radio""s local oscillator signal, or the switching amplifier signal, or both, are compared to the standards. Suitable circuitry then modifies the switching amplifier signal to keep the switching amplifier signal far enough away from the tuned AM radio station and the local oscillator and thereby avoid the problem of interference. The invention provides means for monitoring the local oscillator and the switching signal and selecting a switching oscillator signal that has a frequency which is neither a harmonic of the local oscillator nor the tuned AM radio station. The invention either generates the switching signal from the local oscillator or selects another oscillator with a frequency that is not a harmonic of the local oscillator or tuned AM radio station.
The class D amplifier controlled by the divided local oscillator signal in each of these embodiments may be any suitable amplifier, including a self oscillating pulse width modulator with an integrator with feedback from the output of the amplifier and a comparator coupled to the output of the integrator. The output of the modulator is coupled to a bridge gate driver that controls the power to a MOSFET bridge circuit. The bridge circuit is connected between high and low voltage power busses and has at least two MOSFETs connected in series with each other. The class D amplifier under discussion must have a provision for external control of its switching frequency.
The local oscillator signal is present in all AM radios and is at a frequency of 450 or 455 kHz above the tuned radio station in radios designed to receive the US broadcast band. The local oscillator may be found at different offsets from the tuned station in other nations, but a circuit can be designed as long as the offset is known. The local oscillator can take any periodic form depending on the design of the tuner. Often the local oscillator is a sine wave created by a phase-locked loop circuit.
Those skilled in the art understand that the control concept described in the analog comparator embodiment can be implemented by using any of a very large number of physical products, including, but not limited to, digital devices such as Complex Programmable Logic Devices (CPLDs), Field Programmable Gate Arrays (FPGAs), microcontrollers, semi-custom or custom Application Specific Integrated Circuits (ASICs), and 74xxxx series integrated circuit logic gates. A large number of different analog devices, including resistors, capacitors, inductors, transistors, and field-effect transistors (FETs) may be combined in different ways to implement the analog portion of the algorithm presented here. Future technological advances may produce other physical devices capable of implementing the algorithm. Regardless of the products used for implementation of this algorithm, any implementation of the algorithm is covered in this patent.
Analog Comparators
One embodiment of the invention uses analog comparators and a digital counter. That embodiment takes a local oscillator signal from the AM tuner and uses it to intelligently determine a fixed operating frequency for a class D amplifier. The local oscillator is divided by an integer number N where 2 less than N less than 7 for the US AM broadcast band and the particular class D amplifier for which the system was devised. N varies between three and six inclusive throughout the range of local oscillator frequencies used in an AM tuner. N for any particular local oscillator frequency is chosen so that the frequency and its harmonics resulting from dividing the local oscillator frequency by N are as far as possible from the tuned radio station corresponding to the frequency of the local oscillator.
The analog comparator embodiment provides a method for determining the appropriate value of N based on a pre-determined algorithm. The method is comprised of a set of analog voltage comparators that control a digital divide-by-N circuit. The divide-by-N circuit divides the frequency of the AM local oscillator pulse train by the appropriate value of N.
Digital Comparators
Another embodiment of the invention relies upon a square wave input oscillator signal and a digital circuit for dividing the square wave to a non-interfering frequency. That embodiment takes a local oscillator signal from the AM tuner and uses it to intelligently determine a fixed operating frequency for a class D amplifier. The local oscillator is divided by an integer number N where 2 less than N less than 7 for the US AM broadcast band in the particular application of this control method presented here. N will vary from three to six throughout the range of local oscillator frequencies used in an AM tuner. N is chosen so that the frequency and its harmonics resulting from dividing the local oscillator frequency by N are as far as possible from the tuned radio station corresponding to the frequency of the local oscillator. Keeping the switching harmonics and fundamental away from the tuned radio station""s frequency and the local oscillator prevents electromagnetic interference.
The digital comparator embodiment providing a method for determining the appropriate value of N is described in this document. The method is essentially a digital window comparator comprised of a counter and a latch that serves as an input to three digital magnitude comparators. The magnitude comparators instruct a divide-by-N circuit on the proper value of N by which to divide the local oscillator.
The class D amplifier under discussion must have a provision for external control of its switching frequency. Such an amplifier with an external input would be controlled by the algorithm described in this patent.
Two Loop
A third embodiment is a two loop digital comparator circuit. It takes a local oscillator signal from the AM tuner and uses it to intelligently determine a fixed operating frequency for a class D amplifier. The local oscillator is divided by an integer number N where, for the particular amplifier used, 2 less than N less than 7 for the US AM broadcast band. N varies throughout the range of local oscillator frequencies used in an AM tuner. N is chosen so that the frequency and its harmonics resulting from dividing the local oscillator frequency by N are as far as possible from the tuned radio station corresponding to the frequency of the local oscillator.
An algorithm for determining the appropriate value of N is described in this document. The algorithm is essentially a pair of frequency comparators that compare the local oscillator frequency with both a maximum frequency and a previously detected frequency. The frequency comparators instruct a divide-by-N circuit on the proper value of N by which to divide the local oscillator.
The class D amplifier under discussion must have a provision for external control of its switching frequency. Such an amplifier with an external input would be controlled by the algorithm described in this patent.
Selected Clocks
A fourth embodiment relies upon selecting one of a plurality of clocks or oscillators based upon a comparison of the local oscillator to the switching frequency of the class D amplifier. It provides a circuit and a method that prevents electromagnetic interference from class D amplifiers from interfering with AM radios located in the same chassis as the class D amplifiers. The method can be used in a variety of consumer electronic audio products such as AM/FM stereo receivers, portable xe2x80x9cboom boxes,xe2x80x9d and personal stereos such as the Sony Walkman. An electronic controller has been developed that controls the switching frequency of a class D amplifier to prevent its switching fundamental and harmonics from interfering with the in-chassis AM radio.
The class D amplifier under discussion must have a provision for external control of its switching frequency. Such an amplifier with an external input would be controlled by the algorithm described in this patent.